I've had my own issues with mounting lights to racks. I don't like placing them on seat or trunk bags, as I use models that are very directional and need to point almost straight back (PB Superflash and PDW Danger zone) On my road bikes, I simply mount it to the seatstay, but panniers will block it on a rack equipped bike.

The Performance Transit rack that is on my wife's bike came with a light mount, but it is designed to mount to the bottom of the top plate and that places the light too low, rubbing against the tire. (I did have to buy the special light mount for a rack, about $5-7) Some careful bending got it into a shape that works. I had this setup on one of my own bikes, but vibration eventually caused a crack in the metal mount right in front of the screw. The mount also rubbed a small hole in the bottom of my rack trunk where it rested on top of it. I'm keeping an eye on my wife's. I'm sure I can find something suitable to replace it at the hardware store when it cracks.
http://bikeforums.net/attachment.php...hmentid=336500


I have two bikes with Blackburn racks, which don't have a top plate, so attaching the light was a little easier. The one I bought recently came with a light mount that wraps around the rack struts. Still had to buy the special clip for my light, but it attached nicely.
http://bikeforums.net/attachment.php...hmentid=336501

My other bike with a Blackburn rack already had the rack when I got the bike. I mounted a PBSF clone with zip ties. I like the horizontal mounting of this. The base section of the light just stays on the bike when I take the light off to change batteries.
http://bikeforums.net/attachment.php...hmentid=336502

Another trick is to use the seatpost or seat stay mount, but placed on the rack itself. Take a bit of electrical tape and an old inner tube to increase the diameter of the rack leg and then clamp the mounting bracket there. Depending on pannier, this actually puts the light out further than the pannier- increases side visibility just a tad (or doesn't block it as much). If I could only take a pic...

I bought a Cateye rear light that came with this mount https://www.shopcateye.com/product/s...-tight-bracket
It worked fine on my Tubus rack.

just happens to be more than one of them.

This. In my experience, taillights take a beating when mounted on a rack, because shock and vibration gets transmitted directly from the rear axle up through the rigid metal rack supports directly to the light (chainstay mounted lights don't fare that well either IME). I've had lights from various manufacturers break off and get lost. But an even bigger problem is lights turning themselves off, probably due to momentary loss of contact in the battery compartment (soft switches are evil on a tail light).

Not really sure how to fix this, other than to mount the light on the seatpost, or even on the rider's helmet, where the shock is less.